Polarizing plates generally have a structure in which a protective film is stacked on one surface or both surfaces of a polarizer formed of a polyvinyl alcohol (hereinafter, referred to as ‘PVA’) resin dyed with a dichroic dye or iodine, using an adhesive. Triacetyl cellulose (TAC) films have mainly been used as protective films for polarizing plates according to the related art, but such TAC films may be easily deformed in high temperature and high humidity environments. Thus, in recent years, various materials of protective films capable of replacing the TAC films have been developed, and for example, methods of using polyethylene terephthalate (PET), a cycloolefin polymer (COP), an acrylic film or the like alone or combinations thereof have been suggested.
However, these protective films are disadvantageous in that sufficient adhesion with respect to aqueous adhesives which have commonly been used for adhesion between PVA films and protective films in the related art may not be secured. In order to solve such disadvantages, a method of forming a primer layer on a surface of polyethylene terephthalate (PET), a cycloolefin polymer (COP), an acrylic film or the like, using a polyurethane primer containing a carboxylic group and increasing adhesion between a protective film and an aqueous adhesive has been proposed.
Meanwhile, since polyethylene terephthalate (PET), a cycloolefin polymer (COP), an acrylic film and the like may have a level of moisture permeability lower than that of TAC and accordingly, in the case of using the aqueous adhesive, moisture contained in the aqueous adhesive may not penetrate through the film, thereby leading to difficulties in the drying thereof. Accordingly, of late, non-aqueous adhesives for replacing aqueous adhesives have been developed.
However, in the case of urethane primers including a carboxylic group, developed according to the related art, they have superior adhesive properties with respect to aqueous adhesives but insufficient adhesive properties with respect to non-aqueous adhesives. Further, since the urethane primers including a carboxylic group may have low levels of water resistance and solvent resistance properties, in a case in which such urethane primers are maintained under high humidity conditions for a relatively long period, adhesion thereof or the like may be deteriorated due to the penetration of moisture.
In addition, the polarizer protective film as described above may include a variety of functional coating layers such as a reflection prevention layer, a hard coating layer and the like, on a surface opposite to a surface to which the polarizer is attached, in terms of reflection prevention, improvements in durability, scratch prevention, an increase in visibility and the like. The functional coating layers may be generally formed by a method of coating a coating composition including a base resin, a solvent, an additive and the like on a protective film and subsequently curing the composition. However, depending on types of the protective film, some cases in which adhesion of the protective film with respect to these functional coating layers may be insufficient may be caused and in such cases, the functional coating layers may be delaminated from the protective film or damaged to thereby degrade functionality of a polarizing plate.
In order to solve these defects, methods of performing a surface treatment such as a plasma treatment, a corona treatment or the like or forming a primer layer on a surface of a protective film have been proposed, but in the case of methods proposed to date, sufficient adhesion between the protective film, in particular, an acrylic protective film and a functional coating layer may not be secured. For example, in the polyurethane primer containing a carboxylic group, when a coating composition including an organic solvent is coated on a primer layer, since the primer layer may be swollen due to the solvent contained in the coating composition, a coated exterior may be defective or the primer layer may be detached from the protective film due to the dissolution of the primer layer in the coating composition.